#book {
  width: 100%;
  height: 100%;
}

.red {
  background: red;
}
.yellow {
  background: yellow;
}
.green {
  background: green;
}
.content {
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  width: 100px;
  height: 100px;
  background: #fff;
  position: absolute;
}
